1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to optical systems for microscopic imaging, inspection and/or lithography applications.
2. Description of the Related Art
Many optical systems and electronic systems are available to inspect surface features of a specimen for defects, including specimens such as a partially fabricated integrated circuit. Defects on such specimens may be relatively small in size and may take the form of imperfections randomly localized on the specimen surface, such as particles, scratches, process variations, repeating pattern defects, and so forth. Techniques and devices for inspecting specimens for these microscopic defects are generally available in the art and are embodied in various commercially available products, including those available from KLA-Tencor Corporation of San Jose, Calif.
The aim of virtually any type of inspection system or technique is to rapidly and efficiently detect defects. With smaller and smaller features on specimen surfaces and the use of new materials and new manufacturing processes, detection of new and finer defects is required. It is also preferable to inspect a specimen surface in as short an amount of time as possible, from loading the specimen to removing it from the inspection position and characterizing the defects. Such speed requirements in the presence of smaller features mandate continuous improvements in the available systems and techniques to accurately and adequately find specimen problems.
Current inspection systems are generally based either on refractive objectives or relatively large sized catadioptric objectives. Systems based on refractive objectives are limited due to imaging performance being typically capped in the presence of light energy transmitted at wavelengths above 365 nm. Designs accommodating shorter wavelengths are possible, but only over a very narrow bandwidth. Design of UV objectives having good correction over fields larger than 100 microns with NAs (numerical apertures) larger than 0.9 has also been difficult.
Inspection systems based on large catadioptric objectives can assist with wavelength limitations seen in refractive objectives. Narrow band catadioptric objective designs with NAs of 0.97 or larger and large field size are possible. These large catadioptric objectives have limitations including high cost, tight manufacturing tolerances, inflexible system architectures, and difficulty controlling contamination associated with DUV illumination.
Many of the aforementioned imaging systems have been constructed of relatively large components, which are difficult or impossible to employ in small environments, such as microscopes and the like. In addition, because of their large size, using multiple objectives in the same system is not practical. However, smaller inspection objectives than have been typically available have been developed. One such ultra-high NA objective is disclosed in U.S. patent application Ser. No. 11/093,850, entitled “Small Ultra-High NA Catadioptric Objective,” inventors Yung-Ho Chuang et al., filed Mar. 29, 2005, U.S. Patent Publication 2006/0158720.
The system disclosed in the '850 application offers certain imaging components and arrangements for inspecting specimens, but the designs disclosed therein cannot simply be used in all environments for imaging different types of specimens. Inspection systems using different geometries, different light sources, with different performance criteria cannot use the designs of the '850 application to accurately and adequately assess specimen flaws under all circumstances.
It would therefore be desirable to have a system for inspecting a specimen that improves upon the systems previously available, and in particular for enabling inspection of specimens such as wafers using a small sized catadioptric objective. It would be particularly desirable to offer systems or designs that may be used under various circumstances and with various components that overcome the imaging issues associated with previously known designs.